1. Field of the Invention
The present invention relates to an optical data recording and/or reproducing apparatus (referred to as, "optical data apparatus" hereinafter) for recording and/or reproducing data on and/or from an optical data storage medium such as an optical disc, optical card, or the like, and more particularly to an optical data apparatus utilizing an optical data storage medium appropriate for the apparatus capable of detecting a tracking error signal to be effectively used particularly for the optical data storage medium on which optical data is recorded at a high density.
2. Description of the Prior Art
An optical memory technology using an optical disc having a pit pattern as a high-density large-capacity data storage medium has been put into practical use while expanding its applications in the forms of digital audio disc, video disc, document file disc, and data file.
As shown in FIG. 25, a conventional optical data storage medium 23 generally has a spiral or concentric continuous guide groove or pit string 24 formed on the surface thereof. The continuous guide groove or pit string 24 is referred to as "track". In order to correctly read the data on the optical data storage medium 23 or correctly record data on the optical data storage medium 23, focusing and tracking controls must be effected to accurately converge a beam from an optical pickup head on the track to trace the same.
Generally, when an optical pickup head having a light source with a wavelength .lambda. of 780 nm and a convergence lens with a numerical aperture of 0.5 are used, the track pitch on the optical data storage medium 23 is actually 1 to 2 .mu.m. However, since it is difficult to depict the track in exact size, the track 24 on the optical data storage medium 23 is depicted schematically as shown in FIG. 25.
FIG. 26 shows a construction of a conventional optical pickup head. In order to effect the focusing and tracking control, a tracking error signal is detected by the optical pickup head 128 to drive an actuator 192 for controlling the position of a converging lens 108 of the optical pickup head or the position of the optical system in accordance with the tracking error signal. The optical system includes a light source 101 and a beam splitter 103. A representative tracking error signal detection method is the sample servo method. The above-mentioned method is disclosed, for example, in the U.S. Pat. Nos. 3,919,697 and 4,553,228.
FIG. 27 is an enlarged view of a part of a track on an optical data storage medium in accordance with the sample servo method. In the present case, it is assumed that the n-th track is designated by Tn, the (n-2)th track is Tn-2, . . . , and the (n+2)th track is Tn+2. On each of the tracks Tn-2 through Tn+2, there is a cyclical combination of a clock pattern area (referred to as "CPA" in the figures), a servo pattern area (referred to as "SPA" in the figures) and a data area. In the clock pattern area, there is formed a pattern for generating timing signals Sa1 and Sa2 representing a timing for sampling a signal obtained from the servo pattern area. In the servo pattern area, there is formed a pattern for generating a servo signal (i.e., tracking error signal), where the pattern is formed as staggered apart from the track by a distance of .+-.p/4 (p: track pitch). In the data area, there is formed a pattern for storing optical data.
FIG. 28 shows a conventional construction of a circuit arrangement for generating a tracking error signal.
Referring to FIGS. 26 and 28, a beam 14 fed by the optical pickup head 128 is converged on the optical data storage medium 23, and then received by a photodetector 13. An electric signal from the photodetector 13 is subjected to a current-to-voltage conversion in an current-to-voltage (I-V) converter 15 to be then transmitted to a phase-locked loop (referred to as, "PLL" hereinafter) circuit 16 and also transmitted to sample and hold circuits 18 and 19.
The PLL circuit 16 generates a clock signal CLK synchronized with a signal obtained from the pattern recorded in the clock pattern area of the optical data storage medium 23. The clock signal CLK is transmitted to a trigger generator circuit 17 to generate timing signals Sa1 and Sa2 representing the timings corresponding to the positions of the patterns P1 and P2 formed in the servo pattern area of the optical data storage medium 23. The sample and hold circuits 18 and 19 sample and hold an output of the I-V converter 15 at the timing of the timing signals Sa1 and Sa2. The signals sampled and held in the sample and hold circuits 18 and 19 are transmitted to a differential operational amplifier 20. After undergoing a differential operation, the resulting signal is sampled and held in a sample and hold circuit 21 to obtain a tracking error signal Tr. The tracking error signal is output from an output terminal 22.
Assuming that the signals held in the sample and hold circuits 18 and 19 are respectively m1 and m2 and the beam from the optical pickup head has a displacement x with respect to the center of the track, the signals m1 and m2 have mutually-antiphase sine waveforms as approximately expressed by the following Equation 1 and Equation 2. The signals m1 and m2 have waveforms as shown in FIGS. 29 (a) and 29 (b). EQU m1=A sin (2.pi.x/p)+B Equation 1 EQU m2=-A sin (2.pi.x/p)+B Equation 2
In Equations 1 and 2, A represents an amplitude, and B represents a DC component.
The signal md sampled and held in the sample and hold circuit 21, i.e., a tracking error signal Tr is expressed by the following Equation 3 and has a waveform as shown in FIG. 29 (c). EQU md=m1-m2=2A sin (2.pi.x/p) Equation 3
The generated tracking error signal Tr is used as a control signal for driving the actuator which controls the beam from the optical pickup head 128 so that the beam is positioned in the desired place on the optical data storage medium 23.
However, when the track pitch p is reduced in order to record a greater amount of data in an optical data storage medium in the case of the conventional optical data storage medium, the signals m1 and m2 as shown in FIG. 29 are gradually reduced in amplitude. Consequently, the tracking error signal md is reduced in amplitude, which results in a relatively increased amplitude of noise contained in the tracking error signal md. Due to the increase of the noise, the tracking operation becomes unstable. When the track pitch p is further reduced to a level at which the inverse number fp of the track pitch p exceeds the spatial cutoff frequency fc of the optical pickup head, the amplitude of each of the signals m1 and m2 becomes 0, for which the tracking error signal md cannot be obtained at all. In other words, the tracking operation cannot be effected, and therefore the data recorded in the optical data storage medium cannot be reproduced.
Assuming that the wavelength of the light source of the optical pickup head is .lambda. and the numerical aperture of the objective lens is NA, the spatial cutoff frequency fc is given by the formula of 2NA/.lambda..
When .lambda.=780 nm and NA=0.45, the spatial cutoff frequency fc is: EQU fc=2NA/.lambda.=1154/mm
The above fact has also resulted in the problem that the tracking error signal cannot be obtained when the track pitch p is not greater than 0.87 .mu.m.
Regarding a tracking error signal detection method and an optical data storage medium capable of detecting a tracking error signal up to a spatial cutoff frequency twice as high as the frequency in the conventional case, the Japanese Patent Laid-Open Unexamined Publications HEI 4-38629 and HEI 4-38633 disclose several examples, however, they have the following problems:
(1) The groove structure of the optical data storage medium is complicated and hardly produced. (See FIG. 7 of the J.P. Laid Open 4-38633)
(2) A plurality of beams must be applied to the optical data storage medium, and therefore the optical system is required to be complicated. (See FIGS. 2 and 5 of the J.P. Laid Open 4-38633)
(3) A great many patterns must be formed in the servo pattern area, and therefore the amount of data allowed to be recorded on the optical data storage medium is significantly reduced. (See FIG. 8 of the J.P. Laid Open 4-38633)